find_in_batches should not mutate its argument

activerecord/lib/active_record/relation/batches.rb

@@ -104,8 +104,8 @@ def find_in_batches(options = {})
         logger.warn("Scoped order and limit are ignored, it's forced to be batch order and batch size")
       end
 
-      start = options.delete(:start)
-      batch_size = options.delete(:batch_size) || 1000
+      start = options[:start]
+      batch_size = options[:batch_size] || 1000
 
       relation = relation.reorder(batch_order).limit(batch_size)
       records = start ? relation.where(table[primary_key].gteq(start)).to_a : relation.to_a

activerecord/test/cases/batches_test.rb

@@ -153,6 +153,12 @@ def test_find_in_batches_should_not_ignore_the_default_scope_if_it_is_other_then
     assert_equal special_posts_ids, posts.map(&:id)
   end
 
+  def test_find_in_batches_should_not_modify_passed_options
+    assert_nothing_raised do
+      Post.find_in_batches({ batch_size: 42, start: 1 }.freeze){}
+    end
+  end
+
   def test_find_in_batches_should_use_any_column_as_primary_key
     nick_order_subscribers = Subscriber.order('nick asc')
     start_nick = nick_order_subscribers.second.nick